This looks like an instance of the so-called double view problem: the same type seen from inside and outside a module ends up being not equal.
OCaml tries to solve it by adding an equation which makes local types point to their external definition.
However, this does not work if the type already has an equation, which is the case if it is a result of using "include".

In this respect, this is just a duplicate of some existing PR, if someone can find it...